This invention relates generally to an electrophotographic copying apparatus, and more particularly, to the heat and pressure fixing of toner images formed on a copy substrate by direct contact with a heated fusing member.
In the process of xerography, a light image of an original to be copied is typically recorded in the form of a latent electrostatic image upon a photosensitive member with subsequent development of the latent image by the application of marking particles commonly referred to as toner. The visual toner image is typically transferred from the member to a copy substrate, such as a sheet of plain paper, with subsequent affixing of the image by one of several fusing techniques. A preferred fusing system applies both heat and pressure to the copy substrate.
In one prior art fusing system, a fuser roll is used which has an outer surface or covering of polytetrafluoroethylene or silicone rubber, the former being known by the trade name Teflon, to which a release agent such as silicone oil is applied, the thickness of the Teflon being on the order of several mils and the thickness of the oil being less than 1 micron. Silicone based oils which possess a relatively low surface energy, have been found to be materials that are suitable for use in a heated fuser roll environment where Teflon constitutes the outer surface of the fuser roll. In practice, a thin layer of silicone oil is applied to the surface of the heated roll to form an interface between the roll surface and the toner images carried on the support material. Thus, a low surface energy layer is presented to the toner as it passes through the fuser nip and thereby prevents toner from offsetting to the fuser roll surface. A fuser roll construction of this type is disclosed in U.S. Pat. No. 3,718,116 assigned to Xerox Corporation.
While heat and pressure fusers of the type discussed above are desirable because of their thermal efficiency, they possess some disadvantages because of their mechanical complexity, cost, long warm-up times and paper wrinkling. A second type of system is known in the prior art which reduces or eliminates these undesirable characteristics. This system utilizes a relatively low mass fuser roll member of the type disclosed, for example, in U.S. Pat. No. 4,689,471 assigned to Xerox Corporation. As disclosed in this patent, a low mass heated fuser roll cooperates with an elongated web member comprising a woven fabric to form an extended fusing area. One end of the pressure web is fixed while the other end is biased into pressure engagement with the fuser roll to form an entrance nip. The pressure web is an enabling feature of this type of system but its effectiveness depends upon several factors such as the type of copy substrate media being used and relative humidity conditions. As an example, certain types of copy media are as subject to stalling or jamming on the leading edge entrance of the fuser entrance nip. The pressure and location of the biasing means if therefore of critical importance. One improvement is disclosed in U.S. Pat. No. 4,860,047 in which a feed roller is introduced at the entrance nip which cooperates with the fuser roll to improve entrance of the copy sheet into the fusing area.
The present invention is directed to a still further improved system whereby a copy sheet entry into the fusing area is effected by causing the fuser web member to move in the direction of copy sheet movement at the critical moment of entry carrying the leading edge of the copy sheet into the entrance nip area. More particularly the invention relates.